The invention pertains to a process for producing ethylene homopolymers and ethylene copolymers in the density range up to 0.930 g/cm3 and in the melt index range between 0.15 and 25 g/10 min (2.16 463 K) with improved process stability and plant available at pressures above 1000 bar and at temperatures up to 603 K in tubular reactors in the presence of free radical-forming initiators, including oxygen and chain regulators, at least one of which has an aldehyde structure. The process is characterized by high monomer conversions and products with good application properties for the film sector.
The use of aldehyde chain regulators in olefin polymerization has long been known and investigated (for example, J. Polym. Sci., Part A-1 (1972), 10(1), 163-168).
The procedures are carried out continuously and as multistep processes. According to DE 1,795,365, ethylene polymers are produced which have a narrow molecular weight distribution in that the propionaldehyde used as the chain regulator is used in the individual reaction zones in certain graded quantity ratios.
U.S. Pat. No. 3,334,081 describes a continuous process for polymerization of ethylene in tubular reactors, wherein the reaction mixture, which also contains a C1-8-aldehyde as a chain regulator, is introduced into the tubular reactor over at least two separate side streams at different points in the reactor and certain product qualities in terms of the melt index can be achieved by using fixed distances between the inflow and outflow of starting materials and products.
The exact and reproducible adjustment of the respective concentrations of initiator, chain regulator, and possibly comonomers in the input gas streams is a basic prerequisite for stable reaction control at high throughput and the preparation of the desired polymer with constant, good quality.
According to DD 276,598, in the manufacturing of ethylene polymers in multizone tubular reactions with at least two side stream feeds by free radical bulk polymerization using oxygen as an initiator at pressures above 80 MPa and temperatures of 373 to 623 K the supply of the input gas streams in the required composition with regard to the oxygen and chain regulator contents takes place in that first, ethylene streams containing chain regulator and oxygen respectively are formed, and by dividing the gas streams and subsequent mixing of the partial streams together in defined quantities and with intermediate pressure return gas at least three input gas streams with defined and reproducible composition are formed, separately compressed to the reaction pressure, and introduced to the reactor.
The relationship of combinations of organic free radical formers, oxygen, and chain regulators in polymerization processes above 1000 bar at very high monomer conversions, however, is also of decisive importance for guaranteeing the plant availability, the process stability, and the fundamental film properties during preparation of the LDPE, depending on the application characteristics.
In the case of combinations of highly active aldehydes as chain regulators (molecular weight regulators) with low-temperature organic peroxides such as perpivalate or pemeodecanoate and oxygen, wherein the organic peroxide and oxygen are always added in respectively the same reaction zone, when defined addition limits of the aldehyde are exceeded, with the aldehyde not being subject to the chain regulator mechanism, non-free radical (ionic) secondary reactions take place, which are characterized as redox and ion transport reactions between all oxygen-containing materials and can preferentially form relatively thermally stable hydroperoxides (especially tert-butyl-hydroperoxide), which in the case of enrichment in the high-pressure circulation, lead to uncontrollable decomposition reactions.
The decisive process here is the concentration and residence time characteristic of the unconsumed organic low temperature peroxide and the acceleration course of its free radical formation in the reactor zone inlet area with specific (low) Reynolds numbers under the chemical influence of the aldehyde and/or its successive products and the oxygen.
Thus in DD 251,261 for start-up processes, a process is described of how spontaneous reaction mixture decompositions can be avoided in that a certain ratio of the volumes of ethylene to oxygen is established.
According to the process of DD 151,453, in addition to oxygen as initiator the peroxide compound tert-butyl-2-ethylhexanoate (t-b-peroctoate) is suggested to reduce the inhibiting effect of oxygen and, by shifting the polymerization reaction toward the beginning of the reactor, to produce a better space-time yield and achieve shorter tubular reactors. The initiator effectiveness in this combination to be sure is excellent and the process stability good, but the overall monomer conversion with any known chain regulator or chain regulator mixture is not satisfactory.
Numerous processes have also become known for polymerization of ethylene without and with comonomers, according to which the attempt is made to combine the advantages of oxygen initiation and organic peroxide compounds by suitable combinations (e.g., U.S. Pat. Nos. 3,781,255; 3,687,867; 3,420,807). For example, according to DE-OS 2,558,266, ethylene is polymerized under high pressure in the presence of at least three initiating agents, wherein aside from oxygen at least one organic initiating agent with a 10-hour half-life temperature of less than 396 K and at least one with a 10-hour half-life temperature of  greater than 403 K are used.
It is also known that peresters, as organic free radical formers, in certain concentration ratios tend to form hydroperoxide with oxygen, and can lead to enrichment at longer residence times. Finally, therefore last but not least process variants are discussed and used generally without molecular oxygen or air, wherein the combinations of aldehyde/high temperature peroxide (ionic) redox systems can form, which can result in a constantly growing free radical deficiency with increasing conversion losses.
In all known publications, the reactor stability criteria are always missing, insofar as chain regulator-initiator combinations with aldehydes, low temperature peroxides with 10-hour half-lives of about  less than 360 K, and oxygen in reactors with cold gas introduction are used, which frequently depending on the respective reactor configuration have a tendency toward labile or unstable process control and/or have low monomer conversions or, in the case of high monomer conversions, are expensive to operate.
The goal of the invention consists of guaranteeing the safe and stable process control with high steady-state continuity and availability in the high-pressure tubular reactor for production of LDPE base materials of low inhomogeneities for the film sector and for compounds under extremely favorable economic manufacturing conditions in the melt index range between 0.15 g/10 min and 25 g/10 min.
The invention pertains to a process technology innovation in the case of high pressure polymerization methods for ethylene in tubular reactor with cold gas guidance for limiting the bulk fraction of (protonic) byproducts and oxidation products produced by auxiliaries, especially of interfering hydroperoxides, as a result of which maximal monomer conversions are produced and excellent properties can be guaranteed in the case of use in the fine, packing, and heavy load film sectors and in the case of cable and piping coating compounds (KBC) on the basis of LDPE. In addition, there is a decisive increase in process stability and plant availability in the case of this free radical polymerization process.
The goal existed, using highly reactive aldehydes as chain regulators, of bringing about a reliable and stable continuous free radical polymerization in multi-zone tubular reaction units using combinations of organic low temperature peroxides and oxygen at maximal conversions. This goal was accomplished by a process with defined input and stability criteria for the fundamental process control and regulation variables.
In this process, chemokinetic characteristic data of thee reactive starting materials (thermal half-lives, concentrations, addition rates) with flow-mechanically relevant characteristic data of the tubular reactor (flow rate) under consideration of the target product quality (melt index, flow coefficient) were coupled in such a way that interfering secondary reactions, especially polar-inductive substitution effects that can lead to formation and enrichment of organic hydroperoxides, are minimized and thus extraordinarily stable process conditions can be achieved.
Here the starting and stability criteria are assigned to a fundamental temperature- and pressure-dependent acceleration field a in each reactor zone with a=uxe2x80x2/t1/2H[peroxide], wherein is the mean flow velocity of the monomer (ethylene) or the reaction mixture (ethylene/polyethylene) at the respective addition site of the organic peroxide in m*sxe2x88x921 and t1/2H [peroxide] is the mean temperature- and pressure-dependent half-life of the organic peroxide at the respective current introduction site into the reactor in seconds.
For a, an applicability range of a=1.0xc2x10.7 m*sxe2x88x922, preferably a=1.0xc2x10.5 m*sxe2x88x922 (fundamental definition range) is established, in which the ratio G/Fz less than 2 (chain regulator limitation) is to be fulfilled, wherein G is the maximal pure aldehyde quantity introduced into each reaction zone in mol/hr and Fz is the flow coefficient of the target product, based on the mean melt index, according to the equation:
Fz=50*[log10(MF1)+1].
Of determining significance for the use in accordance with the invention is the establishment of defined starting and input concentrations of the organic peroxides in each reaction zone, which match the applicability range of/the definition mentioned, with which the initiation reactions starting from a low-temperature region corresponding to the degradation characteristics of primary radicals and chain growth promoting hydrocarbon radicals are generated without the t-butylhydroperoxide formation, which is always possible, leading to enrichment in the high- and intermediate pressure circulation. In the sense of the invention it makes no difference in which chronological peroxide/oxygen molar ratio the free radical control takes place in the respective reactor zone. Thus according to the invention the input concentration c0 of the organic peroxide, which is introduced into the reactor in the form of a suitable peroxide solution, measured in mol/L of pure peroxide to be added, is to be made only so low that the volume flux defined as the ratio G/(Fz*c0) (peroxide concentration limitation) is always below 2 L*hxe2x88x921, wherein all parameters characterizing the volume are based on customary normal pressure (1 bar) and 273 K.
Each individual organic peroxide component that is used is separately subject to the concentration limitation mentioned insofar as the fundamental definition range for the acceleration field a is met at the respective starting temperature in the reactor.
Of lesser importance and irrelevant in accordance with the invention thus are those organic peroxides whose free radical formation speed falls in the same thermal region as that of oxygen and which numerically have a 10-hour half-life of equal to or greater than 360 K.
For guaranteeing stable, steady-state process control at maximal monomer conversions it is essential that the two stability criteria G/Fz less than 2 and G/(Fz*c0) less than 2 are also met in the reaction zones in which, to be sure, organic peroxide is present according to the definition determination, but organic peroxide and oxygen need not be present simultaneously, although organic peroxide and oxygen may be present simultaneously in one or more other zones, preferably in the successor zone. In this way the excess, apparently not control-active aldehyde chain regular fraction does not have a destabilizing effect in the initiation system with organic peroxide and oxygen in the successive zone. In the case of respectively low starting temperatures in the individual reaction zones, using suitable organic peroxides and oxygen, supported by the aldehyde chain regulator, maximal monomer conversions can be achieved with the best process continuity and good optical and mechanical film properties as well as excellent base properties for compounds.
The use of oxygen as a high-temperature peroxide component, according to experience, is substantially more cost-advantageous compared to peroxides and largely saves expensive purging for the sake of limiting the inert fraction.